“And if it looks like a duck and talks like a duck, it should be taxed like a duck.”

From Wikipedia:

“The duck test is a form of abductive reasoning, usually expressed as “If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.”

The test implies that a person can identify an unknown subject by observing that subject’s habitual characteristics. It is sometimes used to counter abstruse arguments that something is not what it appears to be.

The French automaton maker Jacques de Vaucanson created a mechanical duck in 1738. The mechanical duck would quack, move its head to eat grain which it would appear to digest, and after a short time would excrete a mixture that looked and smelled like duck droppings. The irony is that while the phrase is often cited as proof of abductive reasoning, it is not proof, as the mechanical duck is still not a living duck.

Douglas Adams parodied this test in his book Dirk Gently’s Holistic Detective Agency:

“If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family Anatidae on our hands.”

Monty Python also referenced the test in the Witch Logic scene in their 1975 film Monty Python and the Holy Grail, where Sir Bedevere reasons that if ducks float and wood burns, then a person who weighs the same as a duck is made of wood and should be burned as a witch.

The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test:

“If it looks like a duck and quacks like a duck but it needs batteries, you probably have the wrong abstraction.”

Vladimir Vapnik, co-inventor of the support-vector machine and a major contributor to the theory of machine learning, uses the duck test as a way to summarize the importance of simple predicates to classify things. During the discussion he often uses the test to illustrate that the concise format of the duck test is a form of intelligence that machines are not capable of producing.

The philosopher Slavoj Žižek has cited the Marx Brothers’ rewording of the duck test: “He may look like an idiot and talk like an idiot, but don’t let that fool you. He really is an idiot.” The humor of this line lies in its violation of an expected opposite.

A common variation of the wording of the phrase may have originated much later with Emil Mazey, secretary-treasurer of the United Auto Workers, at a labor meeting in 1946 accusing a person of being a Communist:

“I can’t prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I’m certainly going to assume that he is a duck.”

The term was later popularized in the United States by Richard Cunningham Patterson Jr., United States ambassador to Guatemala in 1950 during the Cold War, who used the phrase when he accused Guatemala’s Jacobo Arbenz Guzmán government of being Communist. Patterson explained his reasoning as follows:

“Suppose you see a bird walking around in a farm yard. This bird has no label that says ‘duck’. But the bird certainly looks like a duck. Also, he goes to the pond and you notice that he swims like a duck. Then he opens his beak and quacks like a duck. Well, by this time you have probably reached the conclusion that the bird is a duck, whether he’s wearing a label or not.”

Later references to the duck test include Cardinal Richard Cushing’s, who used the phrase in 1964 in reference to Fidel Castro.

In 2015, a variation of the duck test was applied in the revocation of tax-exempt nonprofit status to Blue Shield of California:

In a startling blow to one of California’s biggest health insurers, the state has revoked the tax-exempt status of Blue Shield of California, forcing the company to pay tens of millions of dollars in back taxes and unleashing a torrent of calls for it to return billions of dollars to customers. The tax board’s action ‘was an acknowledgment of what Blue Shield was already doing, or not doing,’ said Anthony Wright, head of Health Access California, a consumer advocacy group. ‘And if it looks like a duck and talks like a duck, it should be taxed like a duck.’
Also in 2015, Russian Minister of Foreign Affairs Sergey Lavrov used a version of the test in response to allegations that Russian airstrikes in Syria were not targeting terrorist groups, primarily ISIS, but rather Western-supported groups such as the Free Syrian Army. When asked to elaborate his definition of “terrorist groups”, he replied:

“If it looks like a terrorist, if it acts like a terrorist, if it walks like a terrorist, if it fights like a terrorist, it’s a terrorist, right?”
In 2021, a version of the test was used by Singapore’s Minister of Finance Lawrence Wong in response to claims by members of the Progress Singapore Party that their parliamentary motion on free trade agreements was not racist. He said:

“But look, if it looks like a duck, if it walks like a duck, if it quacks like a duck, it is a duck.”.”

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